Regarding a method for concentrating a solution of a water-soluble organic material, the method for concentrating by the use of both distillation and membrane separation, that is, the use of both distillation column, and pervaporation membranes (hereinafter called PV membranes) or vapor permeation membranes (hereinafter called VP membranes) have been applied. Such method for concentrating by the use of such membranes is called pervaporation method (hereinafter called PV method), or vapor permeation method (hereinafter called VP method). In PV method, water permeates through a membrane due to the driving force generated by the difference between the vapor pressure of the water occurred in the solution according to the composition and the temperature of the supplied solution and the pressure of the vapor permeated through the membrane. In VP method, the water permeates through the membrane due to the driving force generated by the difference between the partial pressure of the water in the supplied vapor, and the pressure of the vapor permeated through the membrane. In either method, generally, pressures are reduced on the side of the permeated vapor through the membrane.
FIG. 5 shows a concentrating apparatus for water-soluble organic materials by the use of both distillation column and membrane separator. The concentrating apparatus is composed of distillation column 21, membrane separator 24 for separating water from the mixture of water with water-soluble organic material which is distilled by distillation column 21, condenser 22 for cooling the permeated vapor which has permeated through the membrane of membrane separator 24, and vacuum pump P for reducing the pressure on the side of the vapor permeated through the membrane. The solution at the bottom of distillation column 21 is heated by reboiler 29. For example, in the event of separating the ethanol solution in this apparatus, the vapor highly-containing ethanol is distilled from the top of distillation column 21, as the ethanol solution is fed to distillation column 21. As the distilled vapor is delivered to membrane separator 24, the water contained by the mixture permeates selectively through the membrane, and the concentrated ethanol is flown out of the outlet. The vapor permeated through the membrane is condensed by condenser 22, and returned to distillation column 21.
In Japan Patent Application Kokai Publication No. S63-258602, a method is disclosed for separating the volatile mixture into the permeated portion and non-permeated portion by feeding the evaporator with said volatile mixture, delivering the mixed vapor which is flown out of the top of said evaporator to the one side of the gas permeation membrane, and keeping the pressure lower on the other side of said gas permeation membrane. In this method, the evaporator can be one comprising trays, that is, a distillation column, and the temperature of the mixed vapor which is flown out of the top of the evaporator can be raised by a heater. The solution of the water-soluble organic material can be concentrated by means of such method by feeding the aqueous solution of the water-soluble organic material as the volatile mixture. FIG. 6 shows the concentrating apparatus comprising distillation column 21 and heater 28. As the separating apparatus shown in FIG. 6 is almost the same as the one shown in FIG. 5, except for comprising heater 28 and valve between distillation column 21 and membrane separator 24, only differences are described as follows. The vapor distilled from the top of distillation column 21 is heated up to the desirable temperature by heater 28 prior to being delivered to membrane separator 24. The vapor permeated through membrane separator 24 is flown out of the outlet on the permeated side.
According to this method, the pressure of the vapor delivered to membrane separator 24 is never higher than the operation pressure for distillation column 21, because the pressure of the vapor delivered to membrane separator 24 cannot be increased by heater 28 while it can raise the temperature of the vapor. Therefore, the operating pressure must be increased by raising the temperature at the bottom of distillation column 21 by means of adjusting the heat source for reboiler 29 in order to have a larger driving force of the water to permeate through the membrane of membrane separator 24 by increasing the pressure of the vapor delivered to membrane separator 24. Increase of operating pressures of distillation column causes the following problems: a) construction cost will increase because distillation column 21 requires resistance to higher pressures, 2) the distilling energy cost will increase because the heat source of higher temperature is required for evaporating the high boiling point component at the bottom of distillation column 21, c) the separation efficiency of distillation will decrease because the relative volatility of the water-soluble organic material to the water gets closer to 1.0.